International Journal of Coal Geology

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International Journal of Coal Geology

journal homepage:www.elsevier.com/locate/coal

Petrographic and organic geochemical study of the Eocene Kosd Formation (northern Pannonian Basin): Implications for paleoenvironment and

hydrocarbon source potential

Sándor Körmös

^a,b,⁎

, Achim Bechtel

^b

, Reinhard F. Sachsenhofer

^b

, Balázs Géza Radovics

^c

, Katalin Milota

^c

, Félix Schubert

^a

aUniversity of Szeged, Department of Mineralogy, Geochemistry and Petrology, Egyetem u. 2, H-6722 Szeged, Hungary

bMontanuniversität Leoben, Petroleum Geology, Peter-Tunner-str. 5, A-8700 Leoben, Austria

cMOL Plc, Október huszonharmadika u. 18, H-1117 Budapest, Hungary

A R T I C L E I N F O

Keywords:

Eocene Kosd Formation High volatile bituminous coal Hungarian Palaeogene Basin Organic geochemistry Organic petrology

A B S T R A C T

The Eocene Kosd Formation forms part of the Hungarian Palaeogene Basin. The coal measure of this formation was investigated using an 18 m drill core from borehole W–1. Petrographic and organic geochemical in- vestigations (Rock-Eval pyrolysis, biomarker analysis) were performed in order to characterize the depositional environment, to determine the source of the organic matter within, and to assess the hydrocarbon generative potential.

The presence of marine fossils, high TOC/S ratios and ash yields show that the deposition of the coal measure occurred in a marine delta with individual coal layers accumulating in low-lying, rheotrophic mires. The distribution of land plant-derived biomarkers demonstrates that the peat-forming vegetation was dominated by angiosperms, but the relative contribution of gymnosperms varied through time. In addition to land plants, algae and aquatic macrophytes contributed to the biomass. This dense vegetation established a CO2-limited environment forcing aquatic plants to utilise HCO3−during photosynthesis. The marine environment, as well as the predominance of carbonate rocks in the hinterland, caused slightly alkaline conditions, which, together with reduced oxygen availability, stimulated sulphate-reducing bacterial activity and the microbial degradation of plant remains. Consequently, Kosd Formation coal is very rich in sulphur (up to 8.8%). Moreover, the coal contains vitrinite with a strong orange-brownﬂuorescence colour and swells strongly during pyrolysis. These features are typical for coals with marine inﬂuences.

Vitrinite reﬂectance, Tmax, and biomarker proxies indicate that the organic matter is thermally mature and that the Kosd coal reached the high volatile bituminous rank in the deep borehole (~2.6 km depth). Rock-Eval parameters imply that the coal is gas- and oil-prone and reached the maturity threshold critical forﬁrst gas generation and the onset of oil expulsion.

1. Introduction

During the Eocene, the Mesozoic Tethys Ocean decayed into a series of intercontinental seas (Rögl, 1999). This new configuration of land and sea areas modified oceanic circulation and climate (Popov et al., 2001). In the Late Eocene, Europe formed an archipelago and was en- closed by a subtropical sea, where variations in sea level significantly affected the distribution of depositional environments (Sachsenhofer et al., 2018).

The Hungarian and Slovenian Palaeogene Basin is a predecessor of the Pannonian Basin (Tari et al., 1993). During the Eocene,

sedimentation in this basin was characterized by a generally trans- gressive nature, where depocenters shifted northeastwards through time (Báldi and Báldi-Beke, 1985; Kováčet al., 2016). Hence, coal formation started earlier in the area of the present-day Transdanubian Mountains (the Middle Eocene Dorog Coal Formation) than in the North Hungarian Mountains (the Upper Eocene coal-bearing Kosd Formation;

Figs. 1, 2; Báldi-Beke, 2003a, 2003b; Gidai, 1978; Hámor-Vidó and Hámor, 2007). The Kosd Formation includes economic coal seams (Gidai, 1978). The Kosd coalﬁeld (location is shown inFig. 1) is registered in the national coal cadastre of Hungary (MGSH–Mining and Geological Survey of Hungary, 2019), however, mining of this sub-

https://doi.org/10.1016/j.coal.2020.103555

Received 2 April 2020; Received in revised form 1 July 2020; Accepted 5 July 2020

⁎Corresponding author at: University of Szeged, Department of Mineralogy, Geochemistry and Petrology, Egyetem u. 2, H-6722 Szeged, Hungary.

E-mail address:sandor.kormos@geo.u-szeged.hu(S. Körmös).

Available online 15 July 2020

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bituminous coal ended in the 1930s (Némedi Varga, 2010).

Bechtel et al. (2007)performed an exhaustive geochemical study on the Middle Eocene Dorog Coalﬁnding that the seam originates from a topogenous mire and evolved within a peneplane coastal area covered with eutrophic swamps. The peat-forming vegetation at the time was predominated by angiosperms, which is characteristic of Eocene coals throughout central Europe (Bechtel et al., 2008). In contrast, factors controlling the depositional environment and coal facies of the Upper Eocene Kosd coal have not previously been investigated.

Maceral composition, petrography-based facies indicators and biomarker analysis became essential tools during the last decades for the reconstruction of paleoenvironments and peat-formingfloral changes (e.g.Bechtel et al., 2003, 2007;Gross et al., 2015;Sachsenhofer et al., 2010). Additionally, the stable carbon isotope composition of individual biological markers allows the identification of specific sources (Hayes et al., 1987). Primary producersfix inorganic carbon through photosynthesis, which leads to specific fractionation of carbon isotopes (Diefendorf and Freimuth, 2017;Holtvoeth et al., 2019).Ficken et al.

(1998)found that accumulating organic matter experiences rapid microbial degradation and associated diagenetic changes in the top few centimetres of sediment. Other studies, however, have reported no signiﬁcant changes inδ¹³C during early diagenesis (Li et al., 2017, and references therein). Thermal maturation of organic matter may none- theless aﬀect the carbon isotope composition (Bjorøy et al., 1992;

Clayton, 1991;Rooney et al., 1998). The degree of isotope fractionation depends on the temperature, but the eﬀect is limited to a range of 2‰ as maturity progresses (Clayton, 1991;Schoell, 1984). Therefore, bio- logically controlled isotope compositions can be used to identify pre- cursor sources after diagenesis (e.g. Collister et al., 1994; Freeman et al., 1990;Rieley et al., 1991).

Fig. 1.Location of registered Eocene sub-bituminous coalﬁelds: Kosd–20 (K−20), Varbó–75 (V–75), Noszvaj–1 (Nv–1), and the investigated W–1 boreholes.

Modiﬁed following the digital elevation model and digital coal cadastre of Hungary (Horváth et al., 2005;MGSH–Mining and Geological Survey of Hungary, 2019).

LSBD–Left Side Blocks of Danube.

Fig. 2.Eocene lithostratigraphy across the North-Hungarian range (modiﬁed after Kercsmár et al., 2015; Less, 2015 oral communication). NHM–North Hungarian Mountains and LSBD–Left Side Blocks of Danube. 1–Gánt Bauxite Formation, 2 –Dorog Coal Formation, 3 – Szőc Limestone Formation, 4 – Csolnok Formation, 5–Padrag Marl Formation, 6–Tokod Formation, 7–Kosd Formation, 8–Szépvölgy Limestone Formation, 9–Buda Marl Formation.

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The current study is based on samples from borehole W–1, located about 50 km SE from the abandoned Kosd coalﬁeld (Fig. 1), which drilled the Eocene succession beneath potential Oligocene hydrocarbon source rocks. Whereas the Oligocene source rocks have been investigated by several authors (e.g.Badics and Vető, 2012;Bechtel et al., 2012;Milota et al., 1995;Sachsenhofer et al., 2018), the Eocene Kosd Formation remained largely uninvestigated.

The aims of this study are therefore to enhance understanding of the depositional environment and organic matter sources in this formation, and to estimate the hydrocarbon potential of deep Eocene Kosd coal. To achieve this goal, organic petrological and organic geochemical analyses were performed.

2. Geological setting

A series of sub-bituminous coalﬁelds of Middle Eocene Dorog Coal Formation occur along the Transdanubian Mountains (Hámor-Vidó and Hámor, 2007;Bechtel et al., 2007). Eocene coal in the North Hungarian Mountains belongs to the Upper Eocene Kosd Formation (Fig. 1;Gidai, 1978; Báldi-Beke, 2003a, 2003b). The diachronous development of depocenters followed the gradual northeastward trend of the Eocene transgression (Báldi and Báldi-Beke, 1985). In the western part of the Transdanubian Mountains, the initial transgression took place during the early Lutetian. The next transgressionﬂooded the entire Transda- nubian Range during the late Lutetian (the Dorog Coal Formation;

Fig. 2). In contrast, for the eastern part of the Transdanubian Mountains and the North Hungarian Mountains, the transgression occurred during the Bartonian and Priabonian (the Kosd Formation;Fig. 2;Báldi-Beke, 2003a, 2003b;Kercsmár et al., 2015;Kováčet al., 2016).

The Kosd Formation crops out in small areas throughout the Buda Hills, the Left Side Blocks of Danube and the Bükk Mountains. In the subsurface, it extends towards the south into the GödöllőHills (Fig. 1), where it was encountered by several boreholes (e.g.Bauer et al., 2016;

Palotai and Csontos, 2010).

The Kosd Formation unconformably overlies the Mesozoic basement, which belongs to different mega-units. In the Buda Hills and the Left Side Blocks of Danube (Transdanubian Range Unit) the basement is represented by Ladinian to Norian platform carbonates (e.g. the Budaörs Dolomite Formation; the Dachstein Limestone Formation), and cherty basinal carbonates (e.g. the Mátyáshegy Formation;Haas and Kovács, 2012). In the Bükk Mountains (Bükk Unit), in addition to La- dinian to Norian platform carbonates (e.g. the Berva Limestone For- mation; the Kisfennsík Limestone Formation) and basinal carbonates (the Felsőtárkány Limestone Formation), a deep marine Middle to Upper Jurassic sedimentary succession, with debris flows and turbi- dites, are present (the Mónosbél Group;Pelikán, 2005). The Mesozoic strata were uplifted during the Late Cretaceous and as a consequence of the subaerial exposure intense karstification occurred (Haas and Kovács, 2012).

Depending on the palaeotopography of the karst surface, the thickness of the Kosd Formation varies considerably and reaches 244 m in borehole Noszvaj–1 (Nv–1; the location is shown in Fig. 1; Less, 2005). The Kosd Formation was described in detail in two wells (Kosd–20 [K−20]; Varbó–75 [V–75];Fig. 1) byGidai (1978)andLess (2005). In both wells, the Kosd Formation is characterized by two different sedimentary facies. The lower facies contains fossil- and carbonate-free variegated clay with diﬀerent amounts of rock debris, deposited in a terrestrial environment. The overlying sediments include claystone, siltstone with varying carbonate content, marl and thin coal layers (Gidai, 1978;Less, 2005). The thickness of the lower and upper sedimentary sequences is 63 and 12 m, respectively, at V–75 well (Less, 2005) and 9 and 123 m, respectively, at K–20 well (Gidai, 1978). The coal is intercalated into calcareous claystone and marl at V–75 and K–20 wells, respectively, whereas the direct underlying sediments are characterized by varicoloured clay with carbonate debris. The overlying sediments grade into Miliolina-bearing calcareous marl.Gidai (1978)

and Less (2005) noted an upward transition from a freshwater environment, indicated by gastropods (Melanopsis sp.) to a shallow marine, lagoonal environment marked by the presence of nanno- plankton (Isthmolithus recurves) and foraminifers (Quinqueloculina, Nummulites sp.). Moreover, a proximal coastal environment is supported by mangrove vegetation, represented by Nypa palm pollens (Kvaček, 2010, and references therein;Rákosi, 1978). Coaly layers were noted byGidai (1978)andLess (2005)in boreholes K–20 and V–75, but were not investigated in detail.

The Kosd Formation grades upward into shallow marine platform carbonates of the Szépvölgy Limestone Formation (Fig. 2). The presence of the corallinacean algae and the frequently-occurring monospeciﬁc Nummulites fabianiiindicate deposition on the inner shelf, while a diverse fauna including orthophragminas in the upper part of the formation, marks a transition to an outer shelf environment during the deposition of Szépvölgy Limestone (Less, 2005). Continuing basin subsidence caused deposition of the shallow bathyal Buda Marl For- mation, in low-oxic environments across the Eocene-Oligocene transition (Ozsvárt et al., 2016). The Tard Clay Formation accumulated in oxygen-depleted conditions (Bechtel et al., 2012;Ozsvárt et al., 2016), and the Kiscell Clay Formation was deposited in a more oxygenated environment (Bechtel et al., 2012). A sequence of siliciclastic and carbonate platform sediments terminates the Palaeogene succession, which is followed by thick Neogene deposits (Less, 2005; Kercsmár et al., 2015).

The coal measures of the Kosd Formation in the former Kosd coal- ﬁeld (Fig. 1) are 5 to 32 m thick and include three seams (Kubacska, 1925; Gidai, 1978; Némedi Varga, 2010). The lower seam is 0.5 to 2.5 m thick and was exploited between 1904 and 1931 via an under- ground mine approximately 130 m below the surface. The coal reaches the sub-bituminous stage in the shallow mine. Based on contemporary data (Papp, 1913), the coal includes 3–4 wt% moisture and the ash yield varies from 6 to 20 wt%. The elemental composition of the coal is 56–67 wt% carbon, 4–6 wt% hydrogen, 1 wt% nitrogen and 5–6 wt%

sulphur (Papp, 1913). According toNémedi Varga (2010), the average caloriﬁc value is 19 MJ/kg and the potential reserves were estimated as 15 Mt.

3. Samples and analytical methods

3.1. Samples

Borehole W–1, drilled in the early 2000s in the southern part of the GödöllőHills by MOL Plc. (Fig. 1), penetrated the Kosd Formation from 2415 m to 2791 m. A drill core, representing the upper section of the coal measures in the Kosd Formation, was recovered from 2587 m to 2605 m. In total, 35 samples (Table 1) were obtained from this drill core, each of them was selected based on lithological diﬀerences and is representative of a 20 cm interval. In order to avoid contamination, about 0.5 cm of the outer rim of each core was removed. Moreover, to prevent outlier readings, samples containing pyrite aggregates or no- dules were not selected during sample collection. Pyrite crystals in the macroscopic range were hand-picked from the samples prior to pul- verization.

3.2. Petrographic analysis

Fifteen core samples, including all coal and coaly shale samples as well as selected mudstone samples, were prepared as whole-rock polished blocks for maceral analyses. Each sample was cut perpendicular to the bedding plane, embedded in epoxy resin and polished according to standard procedures (ASTM International, 2015). Organic matter was identified by reflected white- and UV-fluorescent light microscopy using a Leica DM4P microscope equipped with a 50× oil-immersion objective and a point counter equipped with an OptiScan fully auto- mated scanning stage. Maceral analyses were performed using a single

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scan method (Taylor et al., 1998) considering at least 1500 individual points to assess the minimum 500 counts of macerals. The terminology and classiﬁcation of macerals used in this study are based on the ICCP system (ICCP, 1998, 2001;Pickel et al., 2017).

Mean random reﬂectance (%Rr) of telovitrinite was measured according to the methods of Taylor et al. (1998), and was performed under monochromatic (546 nm) light using an Olympus BX41 microscope equipped with a 50× oil-immersion objective and optical stan- dards of Buehler Ltd. Reﬂectance values were processed by image analysis (Taylor et al., 1998).

3.3. Organic geochemical analysis

Powdered rock samples were analysed in duplicate for total sulphur (TS) and total carbon (TC) contents using an ELTRA Helios CS-580A analyser. Samples pre-treated by hot and diluted H3PO4were used to determine total organic carbon (TOC) content. Total inorganic carbon (TIC = TC–TOC) was used to calculate calcite equivalent (CEq) percentages (CEq = TIC × 8.34 [%]). Ash yields were determined according to standard procedures (ASTM International, 2018).

Pyrolysis was carried out in duplicate using a Rock-Eval 6 instru- ment (Lafargue et al., 1998). The S1 and S2 peaks were recorded in order to determine the quantities of free (S1 [mg HC/g rock]) and generated hydrocarbons (S2 [mg HC/g rock]). The temperature of maximum hydrocarbon generation (Tmax [°C]) was recorded and used as a maturity indicator. Derived parameters were also calculated, such as the hydrogen index (HI; HI = S2 × 100/TOC [mg HC/g TOC]),

production index (PI; PI = S1/(S1 + S2) [−]; Espitalié et al., 1977), bitumen index (BI; BI = S1 × 100/TOC [mg HC/g TOC];Killops et al., 1998) and quality index (QI; QI = (S1 + S2) × 100/TOC [mg HC/g TOC];Pepper and Corvi, 1995).

Based on the results of Rock-Eval pyrolysis and the diverse lithologies observed, ten samples (#20 to #29) representing the depth interval between 2599.0 and 2602.4 m were chosen for solvent extrac- tion. The upper- and lowermost samples represent typical low-TOC intercalating lithologies, whereas the others are coaly shale and coal samples together with their adjacent sediments.

Representative portions of powdered rock samples were extracted using a Dionex ASE 350 accelerated solvent extractor. A di- chloromethane solvent was used at conﬁned conditions of 75 °C and 110 bar (full details of this procedure are given inGross et al., 2015).

The saturated compounds were further separated into normal alkanes and branched–cyclic alkanes for compound-speciﬁc isotopic analyses using an activated molecular sieve (Merck, 500 pm pore space), cyclohexane, and a cyclohexane–n-pentane (12:88) solution.

Saturated and aromatic hydrocarbon fractions were analysed using a gas chromatograph equipped with a 30 m DB-5MS fused silica capil- lary column (i.d. 0.25 mm; 0.25μmﬁlm thickness) and coupled to a ThermoFisher ISQ mass spectrometer. The measuring process followed is described inGross et al. (2015).

Stable carbon isotope measurements ofn-alkanes and acyclic isoprenoids were performed on selected samples using a Trace GC in- strument attached to a ThermoFisher DELTA-V isotope ratio mass spectrometer via a combustion interface (GC isolink, ThermoFisher).

Table 1

Bulk geochemical parameters of samples from the Kosd Formation.

ID Depth Lithotype TIC TOC CEq TS Ash yield TOC/TS S1 S2 HI BI QI PI Tmax

# [m; MD] [wt%, db] [mg HC/g rock] [mg HC/g TOC] [°C]

1 2587.10 siltstone 0.6 0.6 4.9 2.5 – 0.2 0.1 0.3 51 – – 0.30 441

2 2587.75 siltstone 2.5 0.5 20.5 2.1 – 0.2 0.1 0.4 75 – – 0.24 446

3 2588.00 siltstone 1.3 0.6 10.9 2.4 – 0.2 0.2 0.4 63 – – 0.31 446

4 2589.20 siltstone 1.3 0.7 10.4 2.8 – 0.3 0.3 0.5 71 – – 0.35 445

5 2590.00 coaly shale 0.8 29.5 6.8 4.0 59.4 7.3 10.0 71.9 243 34 277 0.12 441

6 2590.20 claystone – 7.2 – 1.6 – 4.6 2.5 20.9 289 – – 0.11 443

7 2591.00 siltstone 0.9 1.2 7.5 2.9 – 0.4 0.6 1.0 80 – – 0.39 443

8 2591.80 siltstone 2.8 0.5 23.6 2.3 – 0.2 0.3 0.5 102 – – 0.37 439

9 2592.20 siltstone 4.3 0.5 36.1 1.7 – 0.3 0.1 0.3 62 – – 0.32 443

10 2593.00 siltstone 0.8 0.6 6.6 1.4 – 0.4 0.2 0.3 60 – – 0.31 446

11 2593.70 siltstone 2.3 0.6 18.9 2.5 – 0.3 0.3 0.4 65 – – 0.39 446

12 2594.40 siltstone 0.9 0.4 7.3 2.5 – 0.2 0.1 0.1 36 – – 0.33 439

13 2594.55 coaly shale 0.7 12.7 5.7 3.5 77.9 3.6 3.8 31.9 251 30 280 0.11 441

14 2594.92 siltstone 0.9 1.5 7.8 2.6 – 0.6 0.7 1.0 68 – – 0.40 439

15 2596.80 siltstone 0.9 0.5 7.8 1.7 – 0.3 0.5 0.3 66 – – 0.61 446

16 2597.00 siltstone 0.9 0.5 7.6 2.4 – 0.2 0.1 0.2 40 – – 0.38 438

17 2597.40 siltstone 1.0 0.8 8.4 3.2 – 0.3 0.3 0.5 64 – – 0.36 447

18 2598.30 siltstone 0.8 0.9 6.3 2.4 – 0.4 0.6 0.6 65 – – 0.51 437

19 2598.50 siltstone 0.6 0.8 5.2 2.5 – 0.3 0.6 0.5 54 – – 0.56 432

20 2599.00 siltstone 0.1 1.0 0.5 3.2 – 0.3 0.4 0.4 39 – – 0.47 431

21 2599.90 coal 0.4 78.4 3.6 3.4 5.8 23.0 23.4 186.3 238 30 267 0.11 443

22 2600.00 claystone – 2.9 – 4.8 – 0.6 1.1 3.0 103 – – 0.27 443

23 2600.60 claystone 0.1 2.9 1.0 3.0 – 1.0 1.5 2.9 101 – – 0.34 445

24 2600.95 coal – 45.7 – 8.8 32.1 5.2 14.6 133.1 291 32 323 0.10 441

25 2601.00 siltstone – 3.8 – 4.5 – 0.9 1.8 6.6 171 – – 0.22 448

26 2601.20 coaly shale 0.3 25.0 2.7 5.9 62.0 4.2 9.3 66.1 264 37 301 0.12 440

27 2601.40 siltstone 1.1 3.0 9.3 4.7 – 0.6 1.1 4.9 165 – – 0.19 446

28 2602.00 coaly shale – 21.8 – 4.9 66.2 4.5 6.6 53.8 247 30 277 0.11 438

29 2602.40 siltstone 1.4 1.0 11.9 3.4 – 0.3 0.9 0.8 82 – – 0.52 444

30 2602.60 siltstone 1.1 1.0 8.8 3.1 – 0.3 0.9 0.7 71 – – 0.55 441

31 2603.30 siltstone 1.1 0.7 9.1 3.0 – 0.2 0.5 0.6 90 – – 0.47 441

32 2603.50 siltstone 1.8 0.7 15.4 3.1 – 0.2 0.5 0.7 97 – – 0.41 442

33 2604.10 siltstone 3.5 0.8 29.3 2.1 – 0.4 0.4 0.7 92 – – 0.38 443

34 2604.90 coal 1.1 57.2 9.0 3.4 30.5 16.6 17.2 130.8 229 30 259 0.12 444

35 2605.00 claystone 1.1 0.8 9.4 2.6 – 0.3 0.5 0.9 107 – – 0.38 444

MD–measured depth; db–dry basis; TIC–total inorganic carbon content, TOC–total organic carbon content, CEq–calcite equivalent, TS–sulphur content, TOC/

TS–ratio of total organic carbon versus total sulphur content, S1–free hydrocarbons, S2–hydrocarbons generated during Rock-Eval pyrolysis, HI–hydrogen index, BI–bitumen index, QI–quality index, PI–production index, Tmax–temperature of maximum hydrocarbon generation.

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CO2was injected at the beginning and the end of each analysis in order to perform instrumental calibration. The GC column and temperature program used were the same as above. Stable isotope ratios are reported in delta notation (δ¹³C;Coplen, 2011) relative to the Vienna Pee Dee Belemnite (V-PDB) standard (δ¹³C = [(δ¹³C/δ¹²C)sample/(δ¹³C/

δ¹²C)standard−1]). Delta notation is expressed in parts per thousand or per mil (‰). The analytical error was better than 0.2‰.

4. Results

4.1. Lithology

Based on drill cuttings and well logs (gamma ray, density, porosity) the Kosd Formation is 376 m thick and includes two coal horizons at depths between 2594 and 2649 m (Fig. 3a). The lower and upper coal horizons are about 9 m and 36 m thick, respectively. The sediments Fig. 3.Simpliﬁed lithostratigraphy of the investigated section at the W–1 well. a) Lithologies are assigned based on well logs and drill cuttings. Drill core was recovered from 2587 m to 2605 m. b) Preserved fossils in the siltstone, including echinoid fragment (crossed Nicols) and c) miliolid foraminifer (plane polarized light). d) Location of organic matter rich samples in the higher resolution stratigraphy sequence selected for organic petrology and their maceral composition. Sample depth is given as measured depth (MD).

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between the coal horizons consist of grey sandstone, sandy siltstone, silty claystone, and conglomerate. Non-coal layers within the coal horizons comprise grey, sandy-, argillaceous-, calcareous- and coaly siltstones. Individual coal beds are 0.1 to 2.1 m thick and include coal and coaly shale, and intercalations of siltstone and claystone (Fig. 3a).

The studied drill core (2587–2605 m depth) represents the upper portion of the coal horizon I (Fig. 3a). Its base is located in the upper part of a 2 m-thick coal bed at a depth of 2605 m. The core includes coal layers, which are black, consolidated and hard. Two different lithotypes can be distinguished: (i) a generally bright,finely banded clarain coal, which is brittle, exhibits uneven fracture and occurs as 5 to 50 cm thick layers; and (ii) a stratified coaly shale, including intercalations of thin (< 1 cm) siltstone and claystone layers, in which the thickness varies from 5 to 40 cm. The interseam sediments are dominated by sandy siltstone, which is variably calcareous and argillaceous. Original sedimentary structures are not visible due to bioturbation, however, slumps are recognizable. Two distinct intervals with shell fragments are observed (2596.5–2598.0 m; 2601.5–2602.0 m). Microscopic investiga- tion revealed that the siltstone is rich in fossils, including mollusc, brachiopod, echinoid fragments and miliolid foraminifers (Figs. 3b, c).

4.2. Bulk geochemical parameters

The TOC contents of coal and coaly shale range between 45.7 and 78.4 wt% (avg. 60.4 wt%) and between 12.7 and 29.5 wt% (avg.

22.3 wt%), respectively, whereas the TOC contents of the interseam lithologies vary between 0.4 and 7.2 wt% (avg. 1.3 wt%). Generally, the TOC contents of samples are greater than 2.9 wt% from 2599.9 m to 2602.0 m (#21 to #28), including coal beds and interseam sediments (Table 1). The ash yield of coals varies between 5.8 and 77.9 wt%

(Table 1). The maximum calcite equivalent percentage in coals and intercalating sediments is 6.8 and 36.1 wt% (avg. 4.0 and 10.0 wt%), respectively (Table 1).

The sulphur content of coaly layers ranges between 3.4 and 8.8 wt%

(avg. 4.9 wt%). In the non-coaly intervals, sulphur contents vary between 1.4 and 4.8 wt% (avg. 2.8 wt%). In most cases, below the depth of 2599 m (#20), the sulphur contents are greater than 2.8 wt%, except sample #35. The TOC/TS ratio varies between 3.6 and 23.0 (avg. 9.2) in coaly lithotypes, whereas its value in interseam sediments ranges from 0.2 to 4.6 (avg. 0.5;Table 1).

Tmax values range from 431 to 448 °C (Table 1). The HI ranges from 36 to 291 mg HC/g TOC (Table 1). In Tmax vs. HI plots (Figs. 4a, b), samples can be subdivided into two populations, reﬂecting those with less or more than 150 mg HC/g TOC. The HI of coaly lithotypes varies from 229 to 291 mg HC/g TOC (avg. 252 mg HC/g TOC). The HI in the interseam sediments is from 36 to 289 mg HC/g TOC (avg. 87 mg HC/g TOC), however, higher readings (> 150 mg HC/g TOC) were recorded at samples #6, #25 and #27, which are adjacent to coals (Table 1). BI and QI were calculated for coals and coaly shales, and vary from 30 to 37 mg HC/g TOC and from 259 to 323 mg HC/g TOC, respectively (Figs. 4c, d;Table 1). An important observation noted during Rock-Eval pyrolysis is that coaly samples undergo stronger swelling, i.e. their volume increases signiﬁcantly during heating.

The amount of extractable organic matter (EOM) varies between 31 and 212 mg/g TOC and yields of EOM are usually higher in intercalating sediments (Table 3). EOM is dominated by polar compounds (34–50 wt%), except in samples #20 and #29, where the saturated compounds are prevalent (38 and 43 wt%, respectively). The saturated and aromatic hydrocarbon contents ranging from 5 to 43 wt% and 14 to 28 wt%, respectively. The amount of asphaltene is between 9 and 31 wt

% (Table 3). Aside from the general dominance of polar compounds, coaly lithotypes are found to be characterized by similar average percentages of aromatic hydrocarbons and asphaltenes (avg. 24 and 25 wt

%, respectively), and the quantity of saturated hydrocarbons is low (avg. 5 wt%). In contrast, interseam lithologies are characterized by similar amounts of saturated and aromatic hydrocarbons (avg. 24 and

22 wt%, respectively) and a lower amount of asphaltenes (avg. 13 wt%;

Table 3).

4.3. Maceral composition and vitrinite reﬂectance

Maceral analyses were performed on 15 samples, including coal, coaly shale, and claystone/siltstone (Table 2). Vitrinite group macerals predominate in all samples (Figs. 3d, 5; Table 2). In coals, vitrinite macerals account for between 67 and 99 vol%. Collotelinite (37–46 vol

%), collodetrinite (7–48 vol%) and gelinite (10–21 vol%) are the most abundant vitrinite macerals present. Collotelinite and collodetrinite exhibit an orange-brownﬂuorescent colour under UV light irradiation (Fig. 5). Inertinite macerals are typically in the range of 1 to 3 vol% but reach signiﬁcantly higher values in banded coal samples (26 and 28 vol

%, in #21 and #34, respectively). In banded coal samples, funginite and inertodetrinite are present in addition to fusinite. The percentages of liptinite macerals reach 8 vol% and sporinite and alginite are present in substantial amounts (max. 4 vol%) in some samples.

In claystone/siltstone samples, vitrinite maceral contents vary from 65 to 100 vol%. Inertinite macerals are abundant in the lowermost sample (#35: 34 vol%;Fig. 3d; Table 2). Liptinite macerals, mainly sporinite, are generally rare and do not exceed 2 vol% (Fig. 3d;

Table 2). Framboidal pyrite occurs in substantial quantity throughout all samples, however, the highest contents are observed in the claystone/siltstone samples (Table 2).

Vegetation- (VI) and groundwater inﬂuence indices (GWIAC) were calculated for coaly sediments based onCalder et al. (1991)andStock et al. (2016;Fig. 6;Table 2). VI ranges from 0.7 to 4.8 and GWIACvaries between 2.9 and 39.2 (Table 2).

Mean random vitrinite reﬂectance was found to range from 0.67 to 0.78 (Table 2).

4.4. Molecular composition of hydrocarbons 4.4.1. Straight chain alkanes and isoprenoids

The concentration ofn-alkanes (Tables 3, 4) varies considerably across the investigated section. The compositions of identiﬁedn-alkanes vary within the range ofn-C13ton-C37. Patterns ofn-alkane distribution are characterized by a relatively uniform proportion of mid-chainn- alkanes (n-C21–25; 27–38% of the totaln-alkanes;Fig. 7;Table 4). Coaly lithologies (#21, #24, #26 and #28) and their adjacent sediments (#22 and #27) are dominated by short-chainn-alkanes (n-C15–19; 30–46%;

Fig. 8). In contrast, interseam lithologies are generally dominated by long-chainn-alkanes (n-C27–31; 24–31%; Fig. 8). Furthermore, n-alkane envelopes exhibit a bimodal distribution pattern. The carbon preference index (CPI;Bray and Evans, 1961) is close to one (0.93–1.04,Table 4).

The acyclic isoprenoids, includingi-C13, farnesane,i-C16, norpris- tane, pristane (Pr) and phytane (Ph), are present in great abundance in all samples (Table 3). The ratios of Pr/Ph range from 3.0 to 4.2 (Table 4).

The stable carbon isotopic composition (δ¹³C) ofn-alkanes, Pr and Ph is similar for all studied samples (Fig. 9). With increasing chain length, δ¹³C values of then-C15–19 andn-C21–25 alkanes of selected samples show a decline of about 1‰, respectively. In contrast, long- chain alkanes (n-C27+) show a reversed trend and increasingδ¹³C with carbon number. Pr and Ph exhibit similar isotopic compositions, but Ph is observed to be slightly depleted in¹³C in comparison to Pr (Fig. 9).

4.4.2. Steroids

The concentration of steranes is very low in all samples (Table 3), however, in the saturated hydrocarbon fraction, 5α,14α,17α(H) steranes dominate over 5β,14α,17α(H) isomers, and are present in the C27–C29range. Normalized values of steranes were plotted on a ternary diagram of C27, C28and C29regular steranes (Huang and Meinshein, 1979), showing the relative distribution of steranes indicating organic matter depositional facies (Fig. 10). C29 homologues are found to

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prevail in the samples, followed by C28 and C27steranes. In organic matter-rich samples, C29 steranes predominate in the distributions (Fig. 10; Table 3). The corresponding diasteranes are found in even lower abundance (Table 3), and exhibit similar carbon number distributions as the regular steranes. In the aromatic hydrocarbon fractions, monoaromatic (C29) stigmastanes and triaromatic (C28)

stigmastanes were identiﬁed at very low concentrations.

The stereoisomer ratios of S/(S + R) ofαααC29steranes andββ/

(ββ+αα) of C29steranes were assessed as a thermal maturity parameter. Ratios of the 20S/(20S + 20R) isomers of the 5α,14α,17α(H)–C29steranes range between 0.40 and 0.53. Moreover, ratios ofαββ/(αββ+ααα) isomers of C29steranes range from 0.65 to Fig. 4.Petroleum potential assessment. a) Plot of Tmax vs. HI, showing the maturity and type of kerogen present in the investigated samples (afterEspitalié et al., 1984). b) Plot of Tmax vs. HI, highlighting the increase in HI prior to the onset of oil expulsion (afterSykes and Snowdon, 2002). c) Rank threshold for oil- and gas generation indicated by BI (afterSykes and Snowdon, 2002). d) Rank threshold for oil expulsion determined by QI (afterSykes and Snowdon, 2002).

Table 2

Mineral matter-free (mmf) maceral-, mineral composition, petrographic indices and random vitrinite reﬂectance of the investigated samples.

ID Ct Vd Cd Cg G ΣVIT S A ΣLIP F Sf Fg Ma Mi Id ΣIN Pyr DM VI GWIAC %Rr ± SD

# [vol%, mmf] [vol%]

5 44.0 4.0 33.1 – 9.8 90.9 3.5 3.9 7.4 0.5 1.2 – – – – 1.8 17.4 44.7 1.1 29.8 0.72 ± 0.03

6 – – – – – 100.0 – – – – – – – – – – 25.0 71.0 – – 0.73 ± 0.05

12 – – – – – 98.7 – – 1.3 – – – – – – – 13.2 73.8 – – 0.68 ± 0.05

13 45.9 – 32.2 – 17.4 95.5 1.1 0.5 1.6 1.0 1.8 – – – – 2.8 8.5 27.6 1.4 39.2 0.73 ± 0.04

18 – – – – – 80.4 – – – – – – – – – 19.6 17.3 79.3 – – 0.74 ± 0.06

21 59.1 – 7.4 0.6 0.2 67.4 5.7 1.3 6.9 9.1 12.7 0.3 – 1.1 2.5 25.7 0.1 0.8 4.8 2.9 0.76 ± 0.04

23 – – – – – 90.0 – – – – – – – – – 10.0 13.3 78.7 – – 0.78 ± 0.06

24 36.9 – 40.2 – 20.6 97.7 1.2 – 1.2 0.4 0.6 – – – – 1.0 8.8 24.0 0.9 16.3 0.71 ± 0.02

25 – – – – – 97.0 – – 0.8 – – – – – – 2.3 15.8 76.1 – – 0.78 ± 0.04

26 32.8 0.6 47.7 – 17.0 98.1 0.5 – 0.5 0.6 0.8 – – – – 1.4 8.2 28.9 0.7 31.2 0.68 ± 0.02

27 – – – – – 97.0 – – 1.5 – – – – – – 1.5 14.5 73.2 – – 0.75 ± 0.03

28 38.2 – 40.1 – 20.3 98.7 0.3 – 0.3 0.6 0.4 – – – – 1.0 6.1 50.2 1.0 33.4 0.67 ± 0.04

29 – – – – – 90.9 – – – – – – – – – 9.1 4.4 92.0 – – 0.73 ± 0.06

34 48.5 0.4 9.8 2.8 6.4 67.9 4.1 – 4.1 16.7 4.8 0.1 1.4 1.0 3.8 27.9 8.0 44.7 4.1 15.4 0.75 ± 0.03

35 – – – – – 64.8 – – 0.9 – – – – – – 34.3 14.8 78.2 – – 0.74 ± 0.04

Macerals that were not counted and are in very low percentages (< < 0.1 vol%) are not included in the table, however, resinite, liptodetrinite and cutinite occur in the samples. Ct–collotelinite, Vd–vitrodetrinite, Cd–collodetrinite, Cg–corpogelinite, G–gelinite,ΣVIT–total vitrinite, S–sporinite, A–alginite,ΣLIP–total liptinite, F–fusinite, Sf–semifusinite, Fg–funginite, Ma–macrinite, Mi–micrinite, Id–inertodetrinite,ΣIN–total inertinite, Pyr–pyrite, DM–detrital minerals, VI – vegetation index, GWI_AC – groundwater index, %Rr ± SD – mean random vitrinite reﬂectance ± standard deviation.

VI = (telovitrinite + fusinite + semifusinte + cutinite + sporinite + suberinite + resinite) / (detrovitrinite + inertodetrinite + alginite + liptodetrinite + ex- sudatinite + chlorophyllinite + bituminite;Calder et al., 1991), GWIAC= (gelovitrinite + (ash yield / 2)) / (telovitrinite + vitrodetrinite);Stock et al., 2016).

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0.69 (Table 4).

4.4.3. Hopanoids

Hopanoids are important constituents of the non-aromatic cyclic triterpenoids (Table 3). The measured hopanoid patterns are characterized by the occurrence of 17α,21β(H)–and 17β,21β(H)–type hopanes from C27to C35with C28hopanes being absent. The predominant hopanoids in most samples are 17α,21β–C30and 17β,21α–C29hopane.

The 17α,21β(H)–type homohopanes show a dominant pattern of ex- ponential decrease in peak height with increasing carbon number. In most samples, a series of C32–to C35–benzohopanes was identiﬁed in the aromatic hydrocarbon fractions. In all investigated samples, the ratios of steroids to hopanoids are between 0.06 and 0.15 (Table 4).

The stereoisomer ratio of 22S/(22S + 22R) isomers of 17α,21β(H)–C32hopanes, a thermal maturity parameter, ranges from 0.55 to 0.63 (Table 4).

4.4.4. Sesquiterpenoids, diterpenoids, non-hopanoid triterpenoids Bicyclic sesquiterpenoids are characterized by the occurrence of 8β(H)–homodrimane, 8β(H)–drimane, and 4β(H)–eudesmane.

Furthermore, rearranged drimenes and pentamethyl–trans–decalins are also present in the studied samples. Decalins were identiﬁed as 2,2,4a,7,8–pentamethyl–trans–decalin and 1,2,2,5,5–pentamethyl–- trans–decalin according toNytoft et al. (2009). Diterpenoids consist of bicyclic- (8β(H)–labdane), tricyclic- (pimarane, isopimarane and abietane) and tetracyclic diterpanes (16β(H)–phyllocladane). The tetra- and pentacyclic triterpenoids occur in the analysed samples and the following oleanane, ursane and lupane types were identiﬁed in non-aromatic hydrocarbon fractions: des–A–olean–12–ene, des–A–urs–12–ene, 10β(H)–des–A–oleanane, 10β(H)–des–A–lupane, 10β(H)–de- s–A–ursane, lupane, urs–12–ene, and 18β(H)–oleanane, with a substantial amount of 18α(H)–oleanane.

Aromatic sesquiterpenes are represented by cadalene, calamenene and 5,6,7,8–tetrahydrocadalene. Abietane-type aromatic diterpenoids are present and consist of 1,2,3,4–tetrahydroretene, norsimonellite, si- monellite, and retene. The following aromatic tetra- and pentacyclic triterpenoids are identiﬁed in the aromatic hydrocarbon fractions: tetramethyl–octahydro–chrysenes, trimethyl–tetrahydro–chrysenes, and tri- and tetra aromatic pentacyclic triterpenoids of the oleanane- and ursane-types (tetramethyl–octahydro–picenes, trimethyl–tetrahydro–picenes).

4.4.5. Polycyclic aromatic hydrocarbons

The total ion chromatograms of aromatic hydrocarbon fractions are dominated by naphthalenes and phenanthrenes (Table 3). Naphthalene (N) occurs at low concentrations, however, its alkylated counterparts, methyl- (MN), dimethyl- (DMN) and trimethylnaphthalenes (TMN), are present at a higher amount. The methylnaphthalenes (2–and 1–MN) are present in equal quantities, whereas 1,6–DMN and 1,2,5– and 1,2,7–TMN predominate the dimethyl- and trimethylnaphthalenes, respectively.

Phenanthrene (P), alkylated phenanthrenes (methyl- (MP), dimethyl- (DMP) and trimethylphenanthrenes (TMP)) occur in considerable concentrations in the investigated samples. The 9–, 1–and 2–MP characterize the methylphenanthrenes. The methylphenantrene index (MPI 1;Radke et al., 1982) varies between 0.52 and 0.59 and calculated vitrinite reﬂectance values (%Rc(MPI 1);Radke and Welte, 1983) vary between 0.71 and 0.75%Rc (Table 4). The pattern of Fig. 5.Photomicrographs of coal samples studied. Images were taken under

reﬂected white light (WL) and ultraviolet light (UV) using oil immersion objective. Samples are #5 (a, b), #21 (c, d, e, f), #26 (g, h) and #34 (i, j). A– alginite, Cd–collodetrinite, Ct–collotelinite, Cu–cutinite, F–fusinite, G– gelinite, Id –inertodetrinite, Pyr– pyrite, Qtz – quartz, Re– resinite, S – sporinite and Sf–semifusinite.

Fig. 6.Chart of vegetation- (VI) and groundwater inﬂuence index (GWIAC; after Calder et al., 1991).

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dimethylphenanthrenes is predominated by 1,7–DMP, and the co- eluting 1,3,7–, 1,3,9–and 2,7,10–TMP dominate the trimethylphenanthrenes.

Beside the naphthalenes and phenanthrenes, polycyclic aromatic hydrocarbons identiﬁed include chrysene, alkylated chrysenes, and perylene.

4.4.6. Sulphur-aromatic compounds

The sulphur-containing aromatic compounds identiﬁed include dibenzothiophenes and benzonaphthothiophenes, both of which occur in considerable quantities in the investigated samples (Table 3). Di- benzothiophenes are represented by dibenzothiophene (DBT), methyl- (MDBT), dimethyl- (DMDBT) and trimethyldibenzothiophenes (TMDBT). Methyldibenzothiophenes are characterized by 4–MDBT, co- eluting 3– + 2–MDBT and 1–MDBT, in descending order of concentration. The benzonaphthothiophenes are dominated by benzo[b]

naphtho[2,1]thiophene, whereas benzo[b]naphtho[1,2]thiophene and benzo[b]naphtho[2,3]thiophene occur in lower quantities.

The DBT/P ratios (Hughes et al., 1995) range between 0.74 and 1.04 (Fig. 11;Table 4).

5. Discussion

5.1. Thermal maturity

Vitrinite reﬂectance (0.67–0.78%Rr; Table 2) and Tmax values (431–448 °C; Table 1) indicate that the coals of the Kosd Formation reached high volatile bituminous rank (Taylor et al., 1998) and that the

organic matter is mature, but did not yet reach peak oil window maturity (0.8%Rr, e.g.Peters and Cassa, 1994). Nevertheless, values of Rr and Tmax vary slightly and point to lower values in coaly lithotypes.

The content of liptinite group macerals is limited, however, organic Table 3

Rock extracts, percentage of extract fractions and concentrations of hydrocarbon species of investigated samples in the Kosd Formation.

ID a b c d e f g h i j k l m n o p q r s t u v w

# [mg/g TOC] [wt%] [μg/g TOC]

20 164 38 19 35 9 14973 2082 44 10 221 74 241 17 112 218 300 1 1336 270 1682 244 1050 216

21 31 5 26 50 19 235 78 5 1 56 21 98 2 25 21 113 12 1245 112 515 95 300 87

22 84 10 28 48 13 2018 524 11 2 59 49 321 5 100 54 373 1 3300 319 1809 321 1228 285

23 146 26 21 40 13 11588 2605 55 12 233 257 379 21 88 586 256 56 2348 220 1198 231 994 209

24 57 5 23 46 25 410 158 14 1 60 41 151 8 38 115 131 8 1277 129 630 129 452 102

25 95 13 27 42 18 2497 794 20 4 83 123 297 12 66 239 216 10 2087 190 1007 201 811 169

26 60 6 24 46 24 733 223 14 2 28 35 211 4 38 154 166 3 1451 159 703 161 475 139

27 83 12 24 47 17 2527 571 14 3 96 85 247 8 53 261 276 4 2125 236 1118 235 842 220

28 62 6 21 42 31 639 159 9 1 77 35 142 4 27 108 172 2 1334 159 691 149 497 131

29 212 43 14 34 9 28999 3485 76 21 351 151 254 21 48 908 173 1 843 262 1399 198 835 212

a–extracted organic matter, b–saturated hydrocarbons, c–aromatic hydrocarbons, d–polar compounds, e–asphaltenes, f–n-alkanes, g–isoprenoids, h–steranes, i–diasteranes, j–hopanes, k–sesquiterpenoids, l–aromatic sesquiterpenoids, m–diterpenoids, n– aromatic diterpenoids, o–triterpenoids, p–aromatic triterpenoids, q–naphthalenes, r–alkylated naphthalenes, s–phenanthrenes, t–alkylated phenanthrenes, u–dibenzothiophenes, v–alkylated dibenzothiophenes, w–benzonaphthothiophenes.

Table 4

Concentration ratios of compounds in hydrocarbon fractions of samples from the Kosd Formation.

ID a b c d e f g h i j k l m n o

# [%] [%] [%]

20 22 35 25 1.00 3.30 30 24 46 0.13 0.42 0.65 0.55 0.75 0.89 0.20

21 46 27 12 1.04 4.10 14 23 63 0.06 0.50 0.67 0.57 0.71 0.83 0.17

22 39 31 15 0.97 4.22 20 23 57 0.11 0.53 0.68 0.56 0.73 0.99 0.20

23 18 29 31 0.97 3.52 26 30 44 0.10 0.43 0.66 0.59 0.74 1.03 0.11

24 35 32 17 0.96 3.76 17 17 66 0.11 0.45 0.67 0.59 0.73 0.98 0.16

25 24 33 24 0.97 3.96 26 28 45 0.10 0.44 0.67 0.57 0.72 1.04 0.15

26 34 34 16 0.93 3.63 21 27 52 0.15 0.42 0.66 0.63 0.72 1.00 0.12

27 30 35 19 1.00 3.57 25 28 46 0.06 0.46 0.66 0.59 0.73 0.98 0.10

28 37 34 13 0.96 3.30 18 20 62 0.06 0.49 0.69 0.57 0.72 0.92 0.10

29 17 38 26 1.03 3.01 38 26 36 0.09 0.40 0.66 0.57 0.73 0.74 0.06

a–n-C_15–19/n-C_15–35, b–n-C_21–25/n-C_15–35, c–n-C_27–31/n-C_15–35, d–CPI (Bray and Evans, 1961), e–Pr/Ph, f–C27steranes/Σregular steranes, g–C28steranes/Σ regular steranes, h–C₂₉steranes/Σregular steranes, i–steranes/hopanes, j–20S/(20S + 20R)αααC₂₉steranes, k–αββ/(αββ+ααα) C₂₉steranes, l–22S/

(22S + 22R) C32hopane, m–Rc(MPI 1)(Radke and Welte, 1983), n–DBT/P (Hughes et al., 1995), o–Di- / (Di- + Triterpenoids;Bechtel et al., 2003).

Fig. 7.Normalized distribution of short- (n-C_15–19), mid- (n-C_21–25) and long- chain (n-C_27–31)n-alkanes in the studied samples.

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matter is dominated byﬂuorescent vitrinite (Fig. 5;Table 2). The orange-brownﬂuorescence colour of the vitrinite and the strong swelling during pyrolysis both suggest the predominance of hydrogen-rich

vitrinite (Wilkins and George, 2002). Hence, the vitrinite reﬂectance of coaly lithotypes may be suppressed due to the diagenetic enrichment in hydrogen and sulphur (e.g.Peters et al., 2018cumlit.). Considering the relatively low content of liptinite macerals and extractable organic matter, suppression due to sorption of generated bitumen (Carr, 2000;

Taylor et al., 1998;Wilkins and George, 2002) seems less likely.

Hopane (22S/22S + 22R C32) and sterane (20S/(20S + 20R)ααα C29) isomerisation ratios range from 0.55 to 0.63 and from 0.40 to 0.53, respectively (Table 4). Although this indicates that C32 hopane isomerisation ratios reached their equilibrium value, sterane isomerisation ratios of C29 suggest that steranes did not. This denotes maturities corresponding to the oil window (0.6–0.8%Rr;Mackenzie et al., 1980;

Mackenzie and Maxwell, 1981; Seifert and Moldowan, 1986; Peters et al., 2005). This maturity assessment is supported by ratios of ββ/ (ββ+αα) isomers of the 5α,14α,17α (H)–C29 steranes (0.65–0.69;

Table 4), which are close to their equilibrium value of 0.7 (Seifert and Moldowan, 1986). CPI values close to one (0.93–1.04) further conﬁrm this maturity assessment.

The methylphenanthrene index (MPI) is a well-established parameter for classifying maturity (Radke et al., 1982) and can be used to calculate vitrinite reﬂectance values (Radke and Welte, 1983). The calculated vitrinite reﬂectance values are between 0.71 and 0.75%Rc Fig. 8.Gas chromatograms (total ion current) of saturated hydrocarbon frac-

tion of (a) coal, sample #21 and (b) claystone, sample #23.n-Alkanes are la- belled according to the carbon number. std.–standard (deuterated n-tetra- cosane).

Fig. 9.Carbon isotopic composition ofn-alkanes, pristane and phytane in coal (#24), coaly shale (#26) and interseam sediments (#25, #27).

Fig. 10.Ternary plot of regular steranes showing the normalized abundance of C₂₇, C₂₈and C₂₉sterane isomers and their depositional facies (afterHuang and Meinshein, 1979).

Fig. 11.Cross-plot of pristane/phytane (Pr/Ph) vs. dibenzothiophene/phenanthrene (DBT/P) ratios (afterHughes et al., 1995).